The Michigan Department of Transportation (MDOT) first introduced the Median U-turn Crossover design in the 1960s. Several rural highway corridors had been preserved via policies as early as the 1920s that provided large rights-of-way for wide medians to establish future "super highways". Rural (later suburban) multi-lane highways with bi-directional openings in wide medians were gradually built up in these corridors. By the 1960s, many corridors were largely urbanized and experiencing capacity problems at intersections and at bi-directional crossover locations, largely due to interlocking left turns. To address this concern, engineers devised the Median U-Turn concept, also known as "Michigan U-Turns" or "Michigan Lefts" that converted the bi-directional median openings to one-way crossovers within the wide median. All left-turning traffic was required to use the directional crossovers, both at intersections and at mid-block locations. In urbanized areas, back-to-back crossovers were implemented with an approximate frequency of 1/8-mile spacing to serve numerous development accesses without undue travel time and distance.